Headlight devices need to have a predetermined light distribution pattern specified by road traffic rules or the like. “Light distribution” refers to a luminous intensity distribution of a light source with respect to space. That is, it refers to a spatial distribution of light emitted from a light source. Further, “luminous intensity” indicates the degree of intensity of light emitted by a luminous body and is obtained by dividing the luminous flux passing through a small solid angle in a given direction by the small solid angle.
As one of the road traffic rules, for example, a predetermined light distribution pattern for an automobile low beam has a horizontally long-shape narrow in an up-down direction. To prevent an oncoming vehicle from being dazzled, a boundary (cutoff line) of light on the upper side of the light distribution pattern is required to be sharp. That is, a sharp cutoff line with a dark area above the cutoff line (outide the light distribution pattern) and a bright area below the cutoff line (inside the light distribution pattern) is required.
“Cutoff line” here refers to a light/dark borderline formed when a wall or screen is irradiated with light from a headlight, and a borderline on the upper side of the light distribution pattern. That is, it refers to a light/dark borderline on the upper side of the light distribution pattern. It refers to a borderline on the upper side of the light distribution pattern and between a bright area (inside of the light distribution pattern) and a dark area (outside of the light distribution pattern). Cutoff line is a term used when an irradiating direction of a headlight for passing each other is adjusted. The headlight for passing each other is also referred to as a low beam.
The illuminance is required to be highest at a region on the lower side of the cutoff line (inside the light distribution pattern). The region of highest illuminance is referred to as the “high illuminance region.” Here, “region on the lower side of the cutoff line” refers to an upper part of the light distribution pattern, and corresponds to a part for irradiating a distant area, in a headlight device. To achieve such a sharp cutoff line, large chromatic aberration, blur, or the like must not occur on the cutoff line. “Blur occurs on the cutoff line” indicates that the cutoff line is unclear.
To provide such a complicated light distribution pattern, an optical system configuration using a combination of a reflector, a light blocking plate, and a projection lens is commonly used (e.g., Patent Literature 1). The light blocking plate is disposed at a focal position of the projection lens.
In the headlight disclosed in Patent Literature 1, a semiconductor light source is disposed at a first focal point of the reflector with an ellipsoid of revolution. Light emitted from the semiconductor light source is concentrated at a second focal point. The headlight disclosed in Patent Literature 1 blocks part of the light by the shade (light blocking plate) and then emits parallel light through the projection lens.